The present invention pertains to the transducer art and, more particularly, to a method of, and means for producing a controled preload on a transducer assembly.
Transducer assemblies commonly comprise one or more mating sections which, in final assembly, are clamped together by suitable means. In many transducer configurations, this clamping force can lead to offsets in the transducer sensing elements. A particular example of such a transducer is an accelerometer assembly of the type described in U.S. Pat. No. 3,702,073, invented by Jacobs, issued Nov. 7, 1972, and assigned to the same assignee as the present application. This design is comprised of three primary components, namely, a proof mass assembly which is supported between upper and lower stators. The proof mass assembly includes a movable flapper, or reed, which is cantilevered from an outer annular support member via flexure elements. The flapper and outer annular support member are commonly provided as a unitary, fused quartz piece.
Arcuate capacitor pick-off plates are formed on the upper and lower surfaces of the flapper by means of gold deposition. In addition, upper and lower force-restoring, or torquer coils are mounted to the upper and lower surfaces of the flapper. Each torquer coil is wound on a cylindrical core and is positioned on the flapper such that the longitudinal axis of the cylinder coincides with a line that extends through the center, and is normal to the top and bottom surface of the proof mass assembly.
Each stator is generally cylindrical, having a bore provided through its planar surface. Contained within the bore is a permanent magnet. The bore and permanent magnet are configured such that the torquer coil of the proof mass assembly fits within the bore, with the permanent magnet being positioned within the cylindrical form of the torquer coil. Thus, each stator permanent magnet is in magnetic circuit configuration with a magnetic field as produced by a current flowing through the corresponding torquer coil. Also provided on the planar surface of the stators are capacitive plates configured to form capacitors with the upper and lower capacitor pick-off plates on the proof mass assembly. Thus, movement of the flapper with respect to the upper and lower stators results in a differential capacitance change between the capacitors formed at the upper and lower surfaces of the flapper.
In operation, the accelerometer assembly is affixed to the object to be monitored. Acceleration of the object results in pendulous, rotational displacement of the flapper with respect to outer annular support member and the upper and lower stators. The resulting differential capacitance change caused by this displacement may be sensed by suitable circuity. The circuitry then produces a current which, when applied to the torquer coils, tends to return the flapper to its neutral position. The magnitude of the current required to "restore" the flappers is directly related to the acceleration of the accelerometer.
Accelerometers of the type described in the Jacobs patent are susceptible to bias erros due to stresses created between the stators and the proof mass assembly. These stresses may be transmitted through the outer annular support member to the flexure elements, resulting in a strain thereof and a corresponding deflection of the flapper. This offset is sensed by the associated circuitry, which responds by producing a current through the torquer coils to reposition the flapper. Such correction current represents a bias current error to the accelerometer's output reading.
One source of such offset bias producing stress is found in the means used to clamp, or secure the transducer assembly together. Commonly, the stators and proof mass assembly are held in axial alignment and a bellyband is positioned over the exposed edge of the proof mass and cemented to the upper and lower stators. In one prior art construction, bolts secured the assembly together providing an axial preload, with a bellyband used for lateral support.
The problem with bellyband-type securing means is that they are subject to hoop stress due to the difference in the coefficient of thermal expansion between the stator material, the bellyband material, and the cement used to secure the band to the stators. As a result, a change in temperature of the transducer creates strains between the cement (typically epoxy) and the band and stator elements (typically a metal alloy). Where the assembly utilizes bolts, expansions and contractions of the bolts relative to the transducer assembly similarly create stresses which distort the stators.
In transducer designs, it is desirable, therefore, particularly in high accuracy applications, to provide a means for preloading the transducer elements in a manner that is stable with temperature, time, and other environmental influences.